1. Field of the Invention
The present invention relates generally to the treatment of living biological tissue by optical irradiation, and in particular to a method for stimulating soft, living tissue by laser irradiation.
2. Description of Related Art
Various non-surgical means have been employed in the therapeutic treatment of living tissue. Such techniques have included the application of ultrasonic energy, electrical stimulation, high frequency stimulation by diathermy, X-rays and microwave irradiation. Techniques such as electrical stimulation, diathermy, X-ray and microwave radiation have shown some therapeutic benefit for soft tissues. However, their use has been somewhat limited because of tissue damage caused by excessive thermal effects. Consequently, the energy levels associated with therapeutic treatments involving diathermy, X-ray, microwave and electrical stimulation have been limited to such low levels that little or no benefit has been obtained. Moreover, the dosage or exposure to microwaves and X-ray radiation must be carefully controlled to avoid radiation related health problems. Ultrasonic energy is non-preferentially absorbed and affects all of the surrounding tissue.
Optical energy generated by lasers has been applied for various medical and surgical purposes because of the monochromatic and coherent nature of laser light which can be selectively absorbed by living tissue depending upon certain characteristics of the wavelength of the light and properties of the irradiated tissue, including reflectivity, absorption coefficient, scattering coefficient, thermal conductivity and thermal diffusion constant. The reflectivity, absorption coefficient and scattering coefficient are dependent upon the wavelength of the optical radiation. The absorption coefficient is known to depend upon such factors as interband transition, free electron absorption, grid absorption (phonon absorption), and impurity absorption, which are dependent upon the wavelength of the optical radiation.
In living tissue, water is a predominant component which has an absorption band according to the vibration of water molecules in the infrared range. In the visible range, there exists absorption due to the presence of hemoglobin. Further, the scattering coefficient in living tissue is a dominant factor.
Thus, for a given tissue type, the laser light may propagate through the tissue, substantially unattenuated, or may be almost entirely absorbed. The extent to which the tissue is heated and ultimately destroyed depends on the extent to which it absorbs the optical energy. It is generally preferred that the laser light be essentially transmissive in tissues which are desired not to be affected, and absorbed by the tissues which are to be affected. For example, when applying laser radiation in a tissue field which is wet with blood or water, it is desired that the optical energy not be absorbed by the water or blood, thereby permitting the laser energy to be directed specifically to the tissue to be treated. Another advantage of laser treatment is that the optical energy can be delivered to the treatment tissues in a precise, well defined location and at predetermined, limited energy levels.
Ruby and argon lasers are known to emit optical energy in the visible portion of the electromagnetic spectrum, and have been used successfully in the field of ophthalmology to reattach retinas to the underlying choroidea and to treat glaucoma by perforating anterior portions of the eye to relieve interoccular pressure. The ruby laser energy has a wavelength of 694 nanometers and is in the red portion of the visible spectrum. The argon laser emits energy at 488 and 515 nanometers and thus appears in the blue-green portion of the visible spectrum. The ruby and argon laser beams are minimally absorbed by water, but are intensely absorbed by blood chromogen hemoglobin. Thus, the ruby and argon laser energy is poorly absorbed by non-pigmented tissue such as the cornea, lens and vitreous humor of the eye, but is preferably absorbed by the pigmented retina where it can then exert a thermal effect.
Another type of laser which has been adapted for surgical use is the carbon dioxide (CO.sub.2) gas laser which emits an optical beam which is intensely absorbed by water. The wavelength of the CO.sub.2 laser is 10.6 micrometers and therefore lies in the invisible, far infrared region of the electromagnetic spectrum, and is absorbed independently of tissue color by all soft tissues having a high water content. Thus, the CO.sub.2 laser makes an excellent surgical scalpel and vaporizer. Since it is completely absorbed, its depth of penetration is shallow and can be precisely controlled with respect to the surface of the tissue being treated. The CO.sub.2 laser is thus well adapted for use in various surgical procedures in which it is necessary to vaporize or coagulate neutral tissue with minimal thermal damage to nearby tissues.
Another laser in widespread use is the neodymium doped yttrium-aluminum-garnet (Nd:YAG) laser. The Nd:YAG laser has a predominant mode of operation at its secondary wavelength of 1,320 nanometers in the near infrared region of the electromagnetic spectrum. The Nd:YAG optical emission is absorbed to a greater extent by blood than by water making it useful for coagulating large, bleeding vessels. The Nd:YAG laser at 1,320 nanometers has been transmitted through endoscopes for treatment of a variety of gastrointestinal bleeding lesions, such as esophageal varices, peptic ulcers and arteriovenous anomalies. Such applications of laser energy are thus well adapted where high energy thermal effects are desired, such as tissue vaporization, tissue cauterization, coagulation and as a surgical scalpel.
The following U.S. patents disclose apparatus and method for therapeutic treatment of living tissue by a laser irradiation:
______________________________________ 3,456,651 3,720,213 4,141,362 4,144,888 4,367,729 4,561,440 4,573,465 4,589,404 4,601,288 4,604,992 4,672,969 4,692,924 4,705,036 4,931,053 4,966,144 ______________________________________
Three patents; Dew, 4,672,969; L'Esperance, Jr. 4,931,053 dated Jun. 5, 1990; and Rochkind, et al 4,966,144 dated Oct. 30, 1990, best describe the prior art. This prior art teaches the use of laser energy in certain, specific applications. Dew discusses the use of a laser, specifically, a Nd:YAG type laser operated a secondary wave length of 1,320 nanometers. Dew discloses that Nd:YAG lasers ordinarily operate at 1,060 nanometers. The purpose of the Dew patent is to use a laser to effect wound closure and reconstruction of biological tissue. The laser energy is converted to heat which ultimately breaks down the tissue into collagenous elements which act as "biological glue".
L'Esperance teaches use of two laser beams used in the visible red or low infrared and extremely low power lasers to irradiate tissue. L'Esperance teaches the use either a helium-neon or krypton lasers. The wave length used by L'Esperance is 610-660 nanometers delivering an output of 0.15 milliwatts.
Rochkind dated Oct. 30, 1990, uses either coherent or noncoherent light, but describes a helium-neon laser operating at 632 nanometers with an intensity of 16 milliwatts per square centimeter or an argon type laser generating light at 465 or 520 nanometers with a light intensity of about 40 milliwatts per square centimeter. In addition, Rochkind describes a two-step process in achieving the methods sought by the invention; a first treatment while the tissue is open and exposed during surgery and a second treatment after closure.